Name ______Date______
Astronomy 106
Expansion and age of the Crab nebula
Due Friday March 25
In this lab, you will use two images of the Crab nebula obtained 43 years apart to determine the rate of expansion of the Crab nebula and its age. You will input your measured values from online images into an excel spreadsheet which will perform the required calculations. You will print out and turn in the spreadsheet along with the answers to the questions at the end of the lab. The images and spreadsheet can be found at If the lab fails to work in your browser, try another browser.
Part 1: Scale of the images.
The first task is to determine the scale factor of the images, much like with your solar rotation lab. To do this, notice there are two stars in each image with an arrow pointing at them. As you scroll the mouse over the picture, notice in the lower left hand corner of the browser window some text appears and at the end there are two numbers separated by a comma. These are the image coordinates of whatever is underneath the mouse and are in order (x, y). If you do not see these, try to expand the window. If not, try another browser! I know it works in Firefoxand Internet Explore, but not in Safari. For both images of the Crab nebularecord the x and y position of the star with the arrow pointing at it on the excel spreadsheet. The sheet will take these numbers and automatically calculate the scale factor.
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Part 2: Measuring the knots and calculating the age .
There are links on the main page to each image (1956 Image of the Crab Nebula and 1999 image of the Crab nebula). There is also a link to the excel spreadsheet; you must use this link to download the excel spreadsheet. On each image, the central pulsar and 10 knots are labeled. By measuring the change in the position of each of these knots relative to the central pulsar between the two images, we can calculate the expansion rate and thus the age of the Crab nebula. The pulsar’s position is already entered into the spreadsheet; do not change this. Click on the 1956 image link. Position the cursor between the lines marking each knot, and record in thespreadsheet the x and y position of each knot found in the bottom left hand corner of the page. NOTE: do not double click on the image, if you do, you will get an error message. If that happens use the browser’s back button to return to the image. Return to the main page and click on the 1999 image of the Crab nebula link. Repeat the above process for measuring and recording the positions of the knots. The spreadsheet will convert your picture coordinates to a distance in arcseconds, which is the distance of the knot from the pulsar measured on the sky. It will then use the time difference between the two photographs and determine the velocity the knot has been traveling in arcseconds/year. Using that and the formula velocity X time = distance, it calculates how long ago the SN occurred (assuming the knots started moving when the supernova occurred)and then calculates the age and an uncertainty in the age of the supernova. Use this information to answer the questions below. Print out and turn in the excel sheet and this sheet with the answers to the questions below. When you print the excel sheet, please print it so it fits on one page.
Questions:
What assumption about the knot’s velocity do you have to make for this calculation to be valid?
Using the uncertainty in the age of the Crab nebula, what is the earliest date the supernova event could have occurred?
What is the latest data the supernova could have occurred?
Does your value for the date of the supernova fall between these two extremes?
Does the actual date of 1054 CE fall between these two extremes?
Comment on how good an estimate of the age of the Crab nebula you were able to obtain with the procedure in this exercise. Don’t just say good or bad, tell me why you think it was good or bad.